UV-curable coating compositions and their use for coating polycarbonate molded articles

ABSTRACT

UV-curable coating compositions containing urethane acrylates prepared from hydroxyalkyl acrylates and low-viscosity polyisocyanates having isocyanurate groups, low-viscosity acrylic ester components as reactive diluents, initiators, and optionally solvents and/or other known additives, and their use for coating molded articles of thermoplastic polycarbonates.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the use of special UV-curablecoating compositions based on urethane-modified acrylate resins forcoating molded articles of thermoplastic polycarbonates.

[0003] 2. Description of the Prior Art

[0004] Polycarbonate molded articles are used in a variety of waysbecause of their excellent properties, such as transparency, impact orshock resistance and tensile strength. However, the properties of thesurface of the material, such as its low abrasion and scratch resistanceand the low resistance to solvents, are inadequate for many areas ofapplication.

[0005] In the past, very different solutions have been proposed for theremoval of this drawback (cf e.g. PCT Applications WO 80/00968 and86/04592, U.S. Pat. No. 3,968,305, EP-A 0,274,595 and EP-A 0,274,596).These publications are primarily directed to the coating of moldedarticles with UV-curing lacquer systems. The processes of these priorpublications, however, have a number of serious disadvantages. Inparticular, the adhesion of the lacquers or the scratch resistanceobtained with the lacquers, as the case may be, is not completelysatisfactory. The coating compositions of the two last-mentioned priorpublications cure only under nitrogen as a deactivating medium.

[0006] The coating compositions based on polyacrylate resins havingurethane groups according to EP-A 0,020,344 or according toPCT-Application WO 80/00942 also lead to coatings that still do notfully satisfy the practical requirements with regard to scratchresistance and stability to weathering.

[0007] The coating compositions according to DE-OS 3,134,157 containfairly large amounts of N-vinyl derivatives of linear or cyclic amides.A disadvantage of these compositions is the need to carry out the UVirradiation several times in order to obtain good scratch resistance.Furthermore, a low viscosity can be obtained in this case only by usinglarge amounts of N-vinyl pyrrolidone. However, this reduces theresistance to scratching and to solvents as well as the stability toweathering. In addition, if fairly large amounts of N-vinyl pyrrolidoneare used, the risk exists of the partial solution, whitening and stresscracking of the polycarbonate, as long as the coating is not yet cured.N-vinyl-pyrrolidone is also undesirable with regard to job hygiene.

[0008] The urethane acrylates according to DE-OS 3,819,627 also are usedin combination with N-vinyl compounds, which leads to thedisadvantageous consequences already mentioned.

[0009] The urethane acrylates according to DE-OS 4,021,109 are used incombination with tri- to hexafunctional (meth)acrylates as reactivediluents in order to avoid the said disadvantages of the aforementionedsolvents. However, the stability to weathering of the resulting lacquerfilms is insufficient. This becomes recognizable as a loss of adhesioneven after a relatively short time.

[0010] Finally, DE-OS 3,318,147 is concerned with special urethaneacrylates without any connection to the special problem of coatingpolycarbonate molded articles.

[0011] It is an object of the present invention to provide coatingcompositions for coating thermoplastic polycarbonates which do notsuffer from the disadvantages of the coating compositions of the priorart.

[0012] This object may be achieved with the coating compositions of thepresent invention which are described in more detail below. The coatingcompositions to be used according to the invention differ, e.g. from thecoating compositions according to DE-OS 4,021,109 by the use of aselected low-viscosity polyisocyanate component having isocyanurategroups for the manufacture of the urethane acrylates as well as by theuse of mainly linear bis-acrylates as reactive diluent B).

SUMMARY OF THE INVENTION

[0013] The present invention relates to UV-curable coating compositionscontaining

[0014] A) 20 to 75 wt %, based on the total weight of components A), B)and C), of a reaction product, which is essentially free from hydroxyland isocyanate groups and is prepared from

[0015] A1) one or more hydroxyalkyl acrylates having 2 to 4 carbon atomsin the alkyl group, optionally mixed with up to 30 hydroxyl equivalent%, based on the total weight of component A1), of other alcohols, and

[0016] A2) an polyisocyanate component containing an aliphaticpolyisocyanate which contains isocyanurate groups, is based on1,6-diisocyanatohexane and has an NCO content of 22 to 23.5 wt % and aviscosity at 23° C. of 800 to 1400 mPa.s,

[0017] B) 5 to 80 wt %, based on the total weight of components A), B)and C), of a low-viscosity acrylic ester component containing

[0018] B1) at least 80 wt % of one or more bis-acrylates having amolecular weight below 350 and based on an alkanediol which may containether oxygen atoms and

[0019] B2) up to 20 wt %, based on the weight of component B), of one ormore acrylic esters other than B1),

[0020] C) 0 to 80 wt %, based on total weight of components A), B) andC), of a solvent or solvent mixture and

[0021] D) 0.1 to 10 wt %, based on the total weight of components A), B)and C), of one or more photoinitiators.

[0022] The present invention is also directed to the use of the coatingcompositions for coating molded articles of thermoplasticpolycarbonates.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The coating compositions to be used according to the inventionpreferably contain 25 to 60 wt % of component A), 10 to 75 wt % ofcomponent 8) and 10 to 75 wt % of component C),. wherein thesepercentages are based on the total weight of components A), B) and C).The content of initiator component D) is preferably 1 to 5 wt %, basedon the total weight of components A), B) and C).

[0024] Component A) is selected from reaction products of at least 70%by weight of component A1) and optionally up to 30% by weight ofcomponent A2), wherein these percentages are based on the weight ofcomponent A). Component A) is “essentially free from hydroxyl andisocyanate groups” which means that in the production of the reactionproducts, components A1) and A2) are used at an NCO/OH equivalent ratioof 0.9:1 to 1.1:1, preferably about 1:1.

[0025] Starting compounds A1) are hydroxyalkyl acrylates with 2 to 4carbon atoms in the hydroxyalkyl group or, less preferably, mixtures ofsuch hydroxalkyl acrylates with other compounds having alcoholichydroxyl groups. Suitable hydroxyalkyl acrylates include 2-hydroxyethylacrylate, the isomer mixture formed by the addition of propylene oxideto acrylic acid, or 4-hydroxybutyl acrylate. 2-hydroxyethyl acrylate ispreferred.

[0026] Other suitable hydroxyl compounds include mono- to trihydricaliphatic alcohols having molecular weights of 32 to 400, such asmethanol, ethanol, n-hexanol, isooctanol, isododecanol, benzyl alcohol,ethylene glycol, diethylene glycol, propylene glycol, glycerol andalcohols obtained from these alcohols by alkoxylation.

[0027] Component A2) is selected from polyisocyanates havingisocyanurate groups, which are based on 1,6-diisocyanatohexane(hexamethylene diisocyanate, HDI) and have an NCO content of 22 to 23.5wt % and a viscosity at 23° C. of 800 to 1400 mPa.s. Thesepolyisocyanates are obtained in a known manner by the trimerizing HDIuntil the reaction mixture has an NCO content of 42 to 45, preferably42.5 to 44.5 wt %, subsequently terminating the reaction and removingunreacted HDI by distillation to a residual content of less than 0.5 wt%.

[0028] The manufacture of component A) by reacting starting componentsA1) and A2) can be carried out by known methods, optionally by usingsuitable urethane catalysts. As previously indicated, the reaction iscarried out at an NCO/OH equivalent ratio of 0.9:1 to 1.1:1, preferablyabout 1:1.

[0029] Suitable catalysts for this reaction include tin(II) octanoate,dibutyltin dilaurate and tertiary amines such as dimethylbenzylamine.The reaction can be carried out in the absence of the other componentsor also in the presence of reactive diluent component B) and/or solventcomponent C), provided that these components do not contain H atomsreactive with isocyanate groups, i.e., isocyanate-reactive groups.

[0030] Urethane acrylate A) can be protected from premature andundesirable polymerization before the manufacture of the coatingcompositions according to the invention by the addition of suitableinhibitors and antioxidants, such as phenols and/or hydroquinones, inamounts of 0.001 to 0.3 wt % each, based on the weight of the urethaneacrylate. A very suitable inhibitor is phenothiazine, which is used inthe examples. This inhibitor can be added before, during and/orfollowing the reaction of components A1) and A2).

[0031] Component B) is selected from bis-acrylates of alkanediols,optionally containing ether oxygen atoms, provided that thebis-acrylates have a molecular weight of less than 350. Thesebis-acrylates may be mixed with up to 20 wt %, based on the total weightof component B), of other acrylic esters of tri- to hexafunctionalalcohols. Suitable bis-acrylates include dipropylene glycol diacrylate,tripropylene glycol diacrylate, 1,4-butanediol diacrylate and1,6-hexanediol diacrylate. Suitable acrylic esters of higher alcoholsinclude trimethylolpropane triacrylate, pentaerythritol tetraacrylate orsorbitol hexaacrylate. Component B) preferably exclusively contains1,6-hexanediol diacrylate.

[0032] Suitable solvents C) include C₅-C₈ aliphatic compounds orcydo-aliphatic compounds, fluorinated/chlorinated hydrocarbons, C₁-C₆aliphatic or cycloaliphatic alcohols and solvent systems containing

[0033] a) 50 to 95 wt % of C₁-C₆ aliphatic alcohols, such as methanol,ethanol, propanol, 1-propanol, n-butanol, and 1-butanol, and

[0034] b) 5 to 50 wt % of aliphatic esters, such as ethyl acetate orbutyl acetate.

[0035] Photoinitiators D) are known and include those described in“Methoden der organischen Chemie” (Houben-Weyl), Volume E 20, pages 80ff, Georg Thieme Verlag, Stuttgart 1987. Preferred are benzoin etherssuch as benzoin isopropyl ether, benzil ketals such as benzildimethylketal, and hydroxyalkylphenones such as2-hydroxy-2-methyl-1-phenyl-1-propan-1-one.

[0036] The coating compositions according to the invention can be mixedwith known additives E) such as UV absorbers, fillers, pigments, dyes,thixotroping agents, smoothing agents, flatting agents and levellingagents, which can be used in the customary amounts.

[0037] Components A), B), C), D) and E) can be mixed by customarymethods, e.g., at room temperature or slightly elevated temperatures,preferably with exclusion of the direct action of light.

[0038] The polycarbonate articles can be coated with the coatingcompositions according to the invention by known methods such as byspraying and dipping processes.

[0039] The curing of the coatings is initiated by UV-irradiation, forwhich purpose commercial UV radiation sources are preferably used.

[0040] Thermoplastic polycarbonates, their manufacture and their use forthe manufacture of molded articles are described for example in“Chemistry and Physics of Polycarbonates”, Polymer Rev. Vol. 9,Interscience Publishers.

[0041] Examples of molded articles of thermoplastic polycarbonates whichcan be coated according to the invention are all types of articles ofdaily use, wherein abrasion and scratch resistance as well as stabilityto weathering are required to a particular degree. Examples includeelectrical articles, cover plates, sight screens, safety discs and solidslabs.

[0042] In the following examples all parts and percentages arepercentages by weight, unless otherwise indicated.

EXAMPLES

[0043] The following polyisocyanates are used in Table 1:

[0044] Polyisocyanate 1

[0045] A polyisocyanate prepared by trimerizing hexamethylenediisocyanate at 60 to 65° C. using trimethylbenzylammonium hydroxide asthe catalyst, terminating the trimerization reaction with dibutylphosphate at an NCO content of 43.6 to 44% and removing excesshexamethylene diisocyanate in the short-path evaporator to a residualcontent of<0.5%. NCO content: 23% Viscosity: 1000 mPa · s (23° C.)

[0046] Polyisocyanate 2—Polyisocyanate from mixture M1 of DE-OS4,021,109

[0047] An HDI-based biuret polyisocyanate available form Bayer AG asDesmodur N 3200. NCO content: 23% Viscosity: 3100 mPa · s (23° C.)

[0048] Polyisocyanate 3—Polyisocyanate from mixture M2 of DE-OS4,021,109

[0049] An HDI-based, isocyanurate group-containing polyisocyanateavailable from Bayer AG as Desmodur N 3300. NCO content: 22% Viscosity:3100 mPa · s (25° C.)

[0050] Preparation of urethane acrylates (Component A)

[0051] The polyisocyanates set forth in Table 1 were dissolved in1,6-hexanediol diacrylate. After the addition of 0.02% of dibutyltindilaurate and 0.01% of phenothiazine, based on the total weight ofpolyisocyanate and 2-hydroxyethyl acrylate, 2-hydroxyethyl acrylate wasadded while passing through dry air at 50 to 60° C. The temperature wasmaintained until the NCO content had fallen below 0.1%. TABLE 1Solutions of urethane acrylates A) in acrylic ester B) Solution 2Solution 3 Starting components (%) Solution 1 (Comp) (Comp)Polyisocyanate 1 (A2) 46.0 — — Polyisocyanate 2 — 46.0 — Polyisocyanate3 — — 47.1 2-Hydroxyethyl acrylate (A1) 29.0 29.0 27.9 1,6-Hexanedioldiacrylate (B) 25.0 25.0 25.0 Viscosity (mPa · s) / 23° C.) 4500 65008700

[0052] The solutions of urethane acrylates according to Table 1 wereprocessed to spraying lacquers according to the formulation given inTable 2 and sprayed in one operation at 20 g/m² on plates (145×65×3 mm)of bisphenol A homopolycarbonate (Makrolon AL 2443, Bayer AG). After aflash-off time of 3 minutes at room temperature the coatings were curedby means of a UV irradiation plant available from IST (1 mercuryhigh-pressure UV radiation source, 80 W/cm, 1 m/min belt speed).

[0053] To determine the scratch resistance, the increase in hazinessafter spraying the sample with quartz sand was measured according to DIN52348.

[0054] The weathering resistance was tested by means of acceleratedweathering tests (DIN 53231, Xenotest 1200).

[0055] To evaluate the adhesion of the coating and its weatheringresistance, a cross-cut test was carried out by scratching a grid of 6by 6 lines, each at a distance of 1 mm, in the coating, followed bypressing on and rapidly peeling off an adhesive tape.

[0056] The test results of the coatings are set forth in Table 3. TABLE2 Comparative Comparative Coating Composition (%) Example 1 Example 2Example 3 Solution 1 36.0 Solution 2 36.0 Solution 3 36.0 1,6-Hexanedioldiacrylate (B) 34.9 34.9 34.9 Solvent Mixture* (C) 25.0 25.0 25.02-Hydroxy-2-methyl-1-phenyl-1- 2.0 2.0 2.0 propane-1-one(photoinitiator) Light protective agent** 1.4 1.4 1.4 Light protectiveagent*** 0.7 0.7 0.7

[0057] TABLE 3 Comparative Comparative Example Example Example 1 2 3Scratch Resistance good good good Loss of adhesion after >3000 <1500<1500 weathering (hours)

[0058] Although the invention has been described in detail in theforegoing for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be limited by the claims.

What is claimed is:
 1. An UV-curable coating composition containing A)20 to 75 wt %, based on the total weight of components A), B) and C), ofa reaction product, which is essentially free from hydroxyl andisocyanate groups and is prepared from A1) one or more hydroxyalkylacrylates having 2 to 4 carbon atoms in the alkyl group, optionallymixed with up to 30 hydroxyl equivalent %, based on the total weight ofcomponent A1), of one or more other alcohols, and A2) an polyisocyanatecomponent comprising an aliphatic polyisocyanate which containsisocyanurate groups, is based on 1,6-diisocyanatohexane and has an NCOcontent of 22 to 23.5 wt % and a viscosity at 23° C. of 800 to 1400mPa.s, B) 5 to 80 wt %, based on the total weight of components A), B)and C), of a low-viscosity acrylic ester component containing B1) atleast 80 wt % of one or more bis-acrylates having a molecular weight ofless than 350 and based on an alkane diol which may contain ether oxygenatoms and B2) up to 20 wt %, based on the weight of component B), of oneor more acrylic esters other than B1), C) 0 to 80 wt %, based on totalweight of components A), B) and C), of a solvent or solvent mixture andD) 0.1 to 10 wt %, based on the total weight of components A), B) andC), of one or more photoinitiators.
 2. The coating composition of claim1 wherein component A1) exclusively contains 2-hydroxyethyl acrylate. 3.The coating composition of claim 1 wherein component A2) exclusivelycontains said aliphatic polyisocyanate.
 4. The coating composition ofclaim 2 wherein component A2) exclusively contains said aliphaticpolyisocyanate.
 5. A molded article of thermoplastic polycarbonatecoated with an UV-curable coating composition containing A) 20 to 75 wt%, based on the total weight of components A), B) and C), of a reactionproduct, which is essentially free from hydroxyl and isocyanate groupsand is prepared from A1) one or more hydroxyalkyl acrylates having 2 to4 carbon atoms in the alkyl group, optionally mixed with up to 30hydroxyl equivalent %, based on the total weight of component A1), ofone or more other alcohols, and A2) an polyisocyanate componentcomprising an aliphatic polyisocyanate which contains isocyanurategroups, is based on 1,6-diisocyanatohexane and has an NCO content of 22to 23.5 wt % and a viscosity at 23° C. of 800 to 1400 mPa.s, B) 5 to 80wt %, based on the total weight of components A), B) and C), of alow-viscosity acrylic ester component containing B1) at least 80 wt % ofone or more bis-acrylates having a molecular weight of less than 350 andbased on an alkane diol which may contain ether oxygen atoms and B2) upto 20 wt %, based on the weight of component B), of one or more acrylicesters other than B1), C) 0 to 80 wt %, based on total weight ofcomponents A), B) and C), of a solvent or solvent mixture and D) 0.1 to10 wt %, based on the total weight of components A), B) and C), of oneor more photoinitiators.
 6. The molded article of claim 5 whereincomponent A1) exclusively contains 2-hydroxyethyl acrylate.
 7. Themolded article of claim 5 wherein component A2) exclusively containssaid aliphatic polyisocyanate.
 8. The molded article of claim 6 whereincomponent A2) exclusively contains said aliphatic polyisocyanate.